Lithographically defined etching provides the means to delineate the features on electronic and opto-electronic integrated circuits. Briefly summarized, in this method a resist, usually a photo-sensitive resist (photoresist), is uniformly spread on a semiconductor substrate, which may or may not have previously defined horizontal features or vertical structure. The photoresist is exposed to an optical pattern, developed and removed in the exposed area (in the case of positive resist). Electron beam writing of the photoresist is equivalent. Thereafter, the patterned substrate is exposed to an etching agent. The agent may be an ion beam, a plasma, or, more commonly, a liquid etchant that etches the substrate but not the developed photoresist. Thereby, the exposed area of the substrate is etched but the substrate covered by the patterned resist is not. The patterned developed resist is thus known as a mask. Masks may be formed by other means, such as a metal masked formed by depositing metal over a resist-patterned substrate and then lifting off the resist and the overlying metal.
Conceptually, most etching is assumed to produce nearly vertical sidewalls in the etched holes, the position of the sidewalls being determined by the edge of the mask. Most liquid etchants, in fact, etch the substrate not only in the vertical direction beneath the mask hole, but also to some extent etch horizontally beneath the mask so as to undercut it. The result is a tapered etch, that is, the sidewall is sloped or inclined at a significant angle from the vertical. Some slope is often desirable, such as when a metal interconnect is deposited over the etched substrate in the area of the etched hole. Too sharp a discontinuity in the surface may interrupt the interconnect. This effect is called the coverage problem.
There are several techniques to reliably provide a sloped sidewall. One technique, such as that disclosed by Keyser in U.S. Pat. No. 4,484,978, uses a cantilevered or undercut mask. Two levels of mask material are sequentially deposited over the substrate. The upper level is patterned so as to expose the lower level. A etchant is then applied which etches away not only the exposed lower level mask but also a side portion of the lower level mask underlying the upper level mask. That is, the upper level mask is cantilevered over the substrate with a free space, equal in thickness to the lower level mask, extending away from the originally patterned hole for a distance of 0.1 to 1.0 .mu.m. Thereafter, another etchant is used that etches neither mask level material but which does etch the substrate. (Keyser in fact additionally used an intermediate anisotropic ion beam etch to form a vertical wall trench defined by the cantilever mask, but this step is not material to the present invention.) Because of the diffusion limited etching in the undercut area, the substrate underlying the undercut area is only partially etched relative to the fully exposed hole. The effect is a gradually sloping sidewall generally underlying the undercut area. The slope of the sidewall is not usually defined but appears to be sufficient to overcome the coverage problem. Photomicrographs disclosed by Bonifield et al in U.S. Pat. No. 4,758,305 for a related technique show a minimum consistent slope of about 45.degree. from the horizontal.
Hoepfner et al discloses another two-level mask technique in U.S. Pat. No. 4,092,210. A two-level mask is deposited on an SiO.sub.2 substrate and an aperture is formed extending through both levels to the substrate. Ion etching is performed on the structure. The ions have a very high disintegration or etching rate for the substrate, a moderate rate for the lower level mask and a lower rate for the upper level mask. As long as the upper level mask remains, the substrate is etched vertically. Once the upper level mask has disintegrated, the lower level mask begins to disintegrate beginning at the aperture. Thereby, sloping sidewalls are formed in the SiO.sub.2 substrate. The disclosure is directed to the coverage problem of subsequent layers.
Tamaki et al discloses in U.S. Pat. No. 4,635,090 a two-level mask technique somewhat related to that of Keyser. After formation of the cantilever mask, an anisotropic KOH etch is used. The orientations are such that the KOH forms 55.degree. sidewalls, the upper ends of which are pinned at the interiors of the undercuts. Thereafter, the cantilever mask can be used for vertical ion beam etching. Dautremont-Smith et al disclose a variant of this process in U.S. Pat. No. 4,595,454. They first grew a native oxide to a thickness of 2 nm on InP and thereafter lithographically defined an upper mask layer. Thereafter an etchant was used in which the native oxide was soluble and which anisotropically etched the underlying InP to form a V-groove. They controlled the amount of undercutting by changes in the solubility or the thickness of the native oxide in order to achieve a completely V-shaped groove. This disclosed method is however limited by the poor native oxides in III-V compounds and the fixed anisotropic etching angles, usually 55.degree..
Optical integrated circuits have put new demands on smoothly varying features. Light is guided on such integrated circuits by waveguide structures. Preferably the waveguide will be single-mode and will not have a discontinuity of sufficient size to cause unwanted reflection or radiation because of mode mismatching. However, if the waveguide must transition from one depth in the substrate to another, a gentle slope is required to achieve the above preferred qualities. The 45.degree. or 55.degree. sidewalls achievable in the prior art are not adequately gentle. This requirement is sometimes referred to as an adiabatic transition.
The precise length of the taper necessary to achieve an adiabatic transition depends on the detailed structure of the waveguides and can be difficult to calculate; however, a simple analysis provides an estimate of the order of magnitude of taper length required. To avoid significant coupling to radiation modes, one wants the taper length to be many times the beat length between the guided mode and the radiation modes, with the strongest coupling to the guided mode. See, for example the explanation by A.W. Snyder et al in Chapter 19, entitled "Slowly varying waveguides" appearing in the book Optical Waveguide Theory, (Chapman and Hall, New York, 1983). That beat length is given by .lambda./(N.sub.g -N.sub.r), where .lambda. is the free-space wavelength of the light, N.sub.g is effective refractive index of the guided mode, and N.sub.r is the effective refractive index of the radiation mode. If it is assumed, for example, that N.sub.g is approximately equal to the index of the light bearing GaAs (3.373), and that N.sub.r is approximately equal to the index of the neighboring Ga.sub.0.9 Al.sub.0.1 As (3.327), the required taper length can be estimated to be longer than 22.multidot..lambda.. For an etch depth of .about..lambda., corresponding to a single-mode waveguide, this limit corresponds to a slope angle of 2.6.degree.. Therefore, it is recommended that slope angles not exceed .about.1.degree. (taper length 57 times the etch depth). Slope angles of less than .about.0.01.degree. (taper length 5720 times the etch depth) become impractical for many opto-electronic applications because the slope extends over a large fraction of the chip.
Chang-Hasnain et al have disclosed in commonly assigned U.S. Pat. application, Ser. No. 07/341,634, filed Apr. 21, 1989, U.S. Pat. No. 4,922,508, such a transition for a semiconductor laser formed near the bottom of a V-shaped groove. In order to couple this light to a layer near the top of the groove but exterior thereto, the output end of the groove was gently tapered to a tip in the horizontal plane. The anisotropic etching forming the V-shaped groove produced smaller but proportional cross-sections of the V-shaped groove nearer the tip. This technique is, however, useful only when the V-shaped grooves are an integral part of the device.
Other techniques have been proposed to provide adiabatic transitions. Shadowing from an elevated mask or mechanically moving a mask during deposition provides a gentle slope. Thermal broadening of a diffused waveguide or thermally dependent growth conditions can produce tapering if there is a temperature gradient. However, these techniques are not amenable to complex integrated circuits. Waveguides grown by flame hydrolysis with a moving gas jet may have their thickness varied by synchronizing the jet movement with the changes in the gas composition. However, these techniques are not amenable to complex integrated circuits. It has been suggested that for waveguides of width less than a diffusion length in LPE, the mass of deposited material is independent of the width. Therefore, narrower waveguides will be thicker than wider ones. The transition will be continuously tapered. This technique has limited applicability.